1. Field of the Invention
This invention relates to a light emitting element drive system, and more particularly to a light emitting element drive system adapted to provide for improved modulation characteristic for directly modulating a semiconductor laser, light emitting diode or like light emitting element with a drive current.
2. Description of the Prior Art
Since the semiconductor laser, the light emitting diode and like light emitting elements have merit such as small size, high efficiency, easy direct modulation and so on, they are promising as an optical source of optical communication. At present, optical communication systems employing such a light emitting element are being developed. One problem of the light emitting element is a short lifetime, but there has recently been reported a semiconductor laser having a lifetime of several thousand hours and a light emitting diode of a longer lifetime. Further, it is reported that the lifetimes of a semiconductor laser and a light emitting diode in one estimate from the result of aging in a high temperature atmosphere can be longer than one hundred thousand hours and longer than one million hours, respectively.
Thus, the light emitting elements such as the semiconductor laser, light emitting diode and so on have been rapidly improved and, with such improvements in the light emitting elements, performance of an optical repeater and the like of an optical communication system, which are each formed with such a light emitting element, has also been enhanced, and quality requirements for them have also become severe. Especially, in the case of rivalry against a wire communication system employing a coaxial cable and so on, the optical communication system using light emitting elements, an optical cable, etc. must be constructed economically as much as possible by decreasing the number of parts used for the optical repeater and other devices so as to achieve reduced power consumption.
On the other hand, for modulating the light emitting element such as a semiconductor laser, light emitting diode or the like in response to an input modulation signal, it is necessary to sufficiently grasp the dynamic characteristic of the light emitting element. Heretofore, there have been proposed some solutions to deterioration of an optical waveform which is caused by phenomena such as relaxation oscillation and lasing delay time on the light emitting elements. These problems are encountered in the case of relatively high modulation speeds. But in the case of medium and lower modulation speeds, even if the peak value of a drive current is constant, the level of an optical output waveform does not remain unchanged. Especially in the case of obtaining a continuous optical pulse output, there is observed the phenomenon that its level gradually lowers, and in the case where the pulse width of the pulse drive current is relatively large, a sag occurs in the optical pulse waveform. Where such an optical pulse train is used, for example, in PCM communication, a visual display of received signal is degraded to result in increased error rate. In other words, transmission performance of the optical communication system is deteriorated.